1. Field of the Invention
The present invention relates to a method and apparatus for communicating between a transmitter and a receiver in the presence of harmonic interference. More particularly, the present invention relates to a method and apparatus for providing train-to-wayside communications in the presence of harmonic interference by detecting frequencies having harmonic interference and generating a communication signal having sub-carrier frequencies which are different from the detected frequencies of the harmonic interference.
2. Description of the Related Art
Audio frequency (AF) track circuits are used in mass transit systems for detecting the presence or absence of a train within a track circuit block, and for automatic train control by inductively coupling control signals from the track circuit block to a corresponding antenna on a moving train. For advanced automatic train control (ATC) systems, coupling control signals to a moving train requires as high a data rate as possible. Conventional modulation and demodulation techniques, such as rate coded modulation and binary FSK/PSK modulation, are presently used for inductive communications in mass transit systems. However, these modulation,techniques suffer from a relatively low data rate.
Further, AF communication systems used in train, mass transit and people mover systems must operate in the presence of interference. The two major sources of interference are caused by harmonics produced by power system components and by electric propulsion systems. Current switching by a power system generates harmonics having regular intervals in the audio frequency range. These harmonics have a relatively high energy content in the 4-10 kHz range because the switched current is on the order of hundreds of amperes in magnitude at the fundamental frequency of the power system.
Interference caused by power and propulsion systems and its affect on AF communication signaling performance has been considered in detail in publications such as F. Sing et al., "The UMTA Rail Transit EMI/EMC Program: An Overview and Summary," February 1987, Final Report No. UMTA-MA-06-0153-4; F. R. Holmstrom, "Conductive Interference in Rapid Transit Signaling Systems," Vol. 1-2, Final Reports UMTA-MA-06-0153- 85-5, -6, 1985-87; F. R. Holmstrom, "Inductive Interference in Rapid Transit Signaling Systems," Vol. 1-3, Final Reports UMTA-MA-06-0153-85-7, -8, and -9, 1986-87; and V. D. Nene, "Harmonic Characteristics of Rectifier Substations and Their Impact on Audio Frequency Track Circuits," Urban Mass Transportation Administration Report UMTA-MA-06-0025-81-6, 1982. For example, a power substation operating at 60 Hz using a six-pulse rectifier generates harmonic interference at frequencies which are multiples of 360 Hz (6.times.60 Hz), that is, at 360 Hz, 720 Hz, 1080 Hz, . . . , etc. Similarly, a 12-pulse rectifier operating at 60 Hz generates harmonic interference at frequencies which are multiples of 720 Hz (12.times.60 Hz). Further, harmonic interference generated by a propulsion system of a train occurs at multiples of the fundamental propulsion system frequency.
Interference generated by a power substation is coupled into AF track circuits through conductive paths and is, therefore, categorized as conductive interference. Interference from a train propulsion system is coupled in the track circuits inductively, and is categorized as inductive interference. FIGS. 1 and 2 show typical experimentally obtained spectra of conductive interference caused by a power substation and inductive interference caused by a chopper propulsion system of a train, respectively. It should be noted that the conductive interference caused by the power substation exists in the track circuit continuously, while the inductive interference caused by a train propulsion system occurs whenever the train is over a track circuit block.
Presently in train-to-wayside communication in mass transit systems, a plurality of independent channels are established in the audio frequency range for transferring specific information, such as vehicle identity (ID), a speed code, vehicle door control, etc. FIG. 3 shows an example of a portion of a frequency spectrum as used in the BART (Bay Area Rapid Transit) system in San Francisco.
Each independent channel in conventional systems has an independent antenna and AF receiver. In both rate coded modulation and FSK/PSK modulation techniques, and their respective variations, each individual AF receiver includes a fixed frequency bandpass filter coupled to an envelope detector. The filters, which are expensive, are used to separate communication signals of a particular communication channel from the harmonic interference and from other communication channels. Because of the difficulty associated with implementing sharp bandpass filters, the signals of each communication channel must use a bandwidth larger than the Nyquist minimum, thus resulting in an inefficient use of the AF spectrum.
Another limitation associated with the two presently used modulation techniques, which is related to the inefficient use of the AF spectrum, is an associated low data rate. As indicated by the typical interference spectra of FIGS. 1 and 2, consecutive harmonics of power and propulsion systems are located only a few hundred Hertz apart leaving only small "gaps" or "holes" in the AF spectrum for signaling. The conventional modulation techniques provide only a few independent channels within the available gaps between 0-10 kHz. Further, the bandwidths of these independent channels must be narrower than the gaps between the interference harmonics, thus leading to the low data rate for these independent channels.
Since presently available AF communications systems for train-to-wayside communications are not programmable, the previously mentioned fixed-frequency bandpass filters are specially designed to ensure proper selectivity for the individual channels. If an additional signal channel is required, the interference spectrum must be thoroughly investigated for selecting a carrier frequency free from interference. Expensive prototype development follows with testing for verifying that the additional channel is compatible with the associated power and propulsion systems. However, even after ensuring compatibility of the additional communication channel design with the power systems, power and propulsion system components may fail during normal operation, such as, a shorted or open diode in a substation rectifier assembly, or a shorted or open SCR in a chopper propulsion unit. In such an event, the characteristics and frequencies of the interference harmonics within the AF spectrum may change and adversely affect safety and overall reliability of the entire train system.
Most chopper propulsion systems with which the present modulation techniques are compatible use a fixed frequency pulse width modulation (PWM) technique for motor control. In AC drive propulsion, which is becoming more popular in transit systems, the inverter frequency is varied for motor control, thus creating harmonic interference with a correspondingly varying spectrum.
Consequently, there is a need for an efficient modulation technique for train-to-wayside communications which provides a higher data rate in the presence of power system and propulsion system interference than that provided by conventional modulation techniques. Further, there is a need for a modulation technique for train-to-wayside communication techniques which adapts to varying interference conditions and is compatible with future propulsion systems.